Seabed excavation using equipment deployed from surface support vessels is well known. This general technique is routinely used by the marine dredging industry and subsea mining industry. Prior art in these fields is discussed below.
The dredging industry uses a range of excavation tools. The tools are generally suspended from surface support vessels by means of lowering equipment, such as winches or hydraulic elevators, which are located on the deck of the surface support vessel.
There are many different kinds of excavation tools and FIGS. 1 and 2 illustrate two of the most common, a trailer suction dredger and a cutter suction dredger respectively. However the common factor linking the tools in the prior art is that they all steered according to the required excavation trajectory using a combination of the one or both of: i) surface support vessel steering track, ii) winching across seabed using winches located on deck in combination with sheaves on or near the seabed.
The majority of marine dredging activity takes place in shallow water coastal waters. In deep water conditions (over 300 m water depth say) control over the excavation trajectory using these prior art techniques becomes increasingly difficult due to the practical limitations of distance between the surface support vessel and excavation tool on the seabed. In ultra-deep water conditions (over 1,000 m water depth say) efficient control using these techniques is likely to be almost impossible.
The subsea mining industry has developed alternative methods compared to the marine dredging industry. One of the drivers for new methods has been the requirement of this industry to operate in open water further from the coastline, and in rather greater water depths compared to the marine dredging industry. Some of the prior art in this field is discussed below.
Seabed drilling systems are used from drill ships by De Beers for diamond mining in South Africa and Namibia. These drills use the “reverse circulation” technique in which the excavated seabed material is drilled with a very large diameter drill bit and sucked up through the riser pipe 52. Apart from that aspect, the drilling technique is relatively conventional with a drill pipe suspended vertically from a deck-mounted derrick tower 39 and hoist 51. FIG. 3 illustrates this technique.
One of the major drawbacks of this technique is that drilling trajectory is purely vertical and there is no ability to steer the drill bit in a horizontal direction. As such the drill bit must be continually inserted, withdrawn following drilling, and then re-positioned on the seabed so the excavation takes place in a discrete and inefficient “cookie cutter” pattern. The circular nature of the drill bit makes it difficult to join the excavated holes together to make a contiguous excavation.
Another disadvantage is the difficulty in accurately controlling the position of the drill bit. If the mining operation is to be done efficiently, the location of the drill hole must be accurately controlled. Such accuracy can be difficult to achieve, particularly in deep water and in locations where the terrain is rugged (such as in most seabed massive sulphide ore deposits).
A further disadvantage in deep water is due to the fact that the power transfer to the drill bit is mechanically provided by the drill string. The downward force on the drill bit must be provided by the weight of the drill string, and this weight must be accurately controlled. Furthermore, the high torque requirements for the large diameter drill bit must also be provided through the drill string.
As an improvement over the drill ship method, De Beers have also developed a seabed crawler vehicle technique. This is illustrated in FIG. 4. In this technique the excavation tool 54 is mounted on a self propelled crawler vehicle 53 which can drive along the seabed 23. The surface support vessel 18 simultaneously following the track of the vehicle along the seabed 23. In this way the disadvantages of the drill ship technique described above are largely overcome. Firstly, it is possible to make the excavation in a continuous trajectory in the horizontal plane—with obvious advantages in excavation efficiency. Secondly, the location of the excavation tool 54 is controlled locally on the seabed by the location and orientation of the crawler vehicle 53. Thirdly, the crawler vehicle 53 provides its own power source, and it can easily control the interaction forces between the excavation tool and the rock being excavated.
An important distinction between different seabed crawlers is the number of degrees of freedom between the crawler vehicle 53 and the excavation tool 54. If the number of degrees of freedom is low, then the crawler vehicle 53 can make continuous horizontal cuts as it moves about the terrain. If the machine has enough degrees of freedom, it can sit in one place and make 3-dimensional cuts in its local vicinity. However, because a seabed crawler 53 must provide counter-balancing forces, it will be much heavier than a machine of similar production rate and have fewer degrees of freedom.
The seabed crawler vehicle technique has its own disadvantages. One of the key disadvantages being that the surface support vessel 18 is connected to the seabed crawler vehicle 53 by means of a flexible pipe 30 as opposed to a fixed steel pipe. Such flexible pipes are specialist products and extremely expensive to manufacture. Furthermore it is often difficult to adjust the length of this flexible pipe because such pipes are generally manufactured in fixed lengths. This can make it correspondingly more difficult to adjust the seabed crawler technique to account for varying water depths, when compared to techniques which use fixed steel pipes which can be routinely and cheaply made up as required in conventional multi-joint lengths.
Another disadvantage of the seabed crawler vehicle is that it relies on competent and flat seabed terrain for stability during steering. There are notorious examples of severe instability occurring on unstable and sloping seabeds, which has caused the failure of entire mining projects. For example, the terrain around the richest type of mining deposits, seabed massive sulphides, is notoriously rugged.
The third disadvantage of the seabed crawler is that, in order to achieve high production rates, the crawler becomes very large. This is due to the relationship between production rate and excavation tool size, and the further relationship between excavation tool size and crawler size. Because the crawler must provide its own counter-balancing forces, only a fraction of the weight of the crawler can be applied to the excavation tool.
A further example of a method from the subsea mining industry is the use of a vertically-suspended trench cutter type excavator which is hung from a surface support vessel. FIGS. 5 and 6 illustrate a trench cutting system developed by Bauer Maschinen GmbH.
FIG. 6 shows the cutting frame and all the components of the trench cutter type excavator. It consists of a frame 1 with counter-rotating cutter wheels 5 at the base. The cutter wheels 5 inject the cuttings into the suction of a slurry pump 3 which provides power for hoisting the slurry a short distance, tens of meters. In examples of the present invention this would be sufficient to supply the excavation slurry to the bottom of a lift pipe which would have airlift or mechanical pumps or other lift methods to provide most of the power for lifting the excavation slurry potentially thousands of meters to the surface.
An advantage of the vertically-suspended trench cutter compared to the drill ship technique described above is that the section excavated in each cut is rectangular in plan (rather than circular as provided by the drill) and a continuous excavation plan is more efficiently formed. Like the seabed crawler, the trench cutter decouples the power transfer from the surface support vessel and provides it locally. A particular advantage is that the machine is very weight-efficient. The weight of the entire machine stands on the cutters, so the machine can be made much lighter than a seabed crawler with the same production rate. However the key disadvantage is that it is a one-dimensional excavator; there is no included means for creating a horizontal excavation trajectory.
Another technique that has been used is steering the drill bit 61 in a horizontal trajectory by means of tugger lines 62 from the deck of the surface support vessel 18. This technique is shown in Kuntz, U.S. Pat. No. 3,763,580 “Apparatus for Dredging in Deep Ocean,” see FIG. 7. In this technique the tugger lines 62 act through sheave blocks 63 anchored into the seabed 23.
However, it is noted that the use of the tugger lines deployed from the deck of the surface support vessel will have practical limitations and will represent a disadvantage in ultra-deep water as described above in relation to techniques used by the marine dredging industry.
Yu U.S. Pat. No. 7,690,135 “Deep Sea Mining Riser and Lift System,” describes a variation on the seabed crawler vehicle technique that replaces the flexible pipe 30 for lifting the slurry with rigid pipe 38 for the majority of length connecting the seabed crawler vehicle 53 to the surface support vessel 18. This patent describes that the use of flexible pipe 30 is now limited to a far shorter fixed length “jumper” section connecting between the crawler vehicle 53 and bottom of the rigid pipe 38. See FIG. 8.